Touch-sensitive devices may detect touch via several different mechanisms, including but not limited to optical, resistive, and capacitive mechanisms. Some optical touch-sensitive devices detect touch by capturing an image of a backside of a touch screen via an image sensor, and then processing the image to detect objects located on the screen. Such devices may include a light source within the device to illuminate the backside of the display screen such that objects on the screen reflect the incident light toward the image sensor, thereby allowing the object to be detected.
One difficulty that may be encountered with optical touch screen devices involves differentiating between external (ambient) light and light reflected from the light source within the device. Ambient light of sufficient brightness may be mistaken for an object touching the device, and therefore may degrade the performance of the device. Further, the use of a rolling image capture system may introduce additional difficulties with correcting an image for ambient light.